"You can see how Leonardo made the cups transparent, something you can't ordinarily see," he said. "You can also note the state of degradation the painting is in."

That degradation has been the subject of controversy this week.

An Italian newspaper, Corriere della Sera, reported that a sophisticated monitoring and filtration system introduced during a restoration of the chapel in the late 1990s was not preventing particles or substances that could damage the work being brought in by visitors.

The Last Supper was painted by Leonardo da Vinci at the end of the 15th Century and, because of the experimental techniques he used, parts of the masterpiece subsequently peeled off and were badly damaged.

The BBC's Frances Kennedy says the paper reported that equipment monitoring air quality inside the refectory showed that levels of fine particle pollution had tripled in the past two years.

It quoted experts suggesting these particles could settle on the work, eventually creating a dark misty layer.

However, Milan's cultural commissioner said the threat was non-existent.